The brain, the organ of behavior, is like other organs in that selective pressures have shaped both its sructures and its functions. The proposed research will focus on how the brain comes to depend upon specific internal and external stimuli as triggers for adaptive responses. The behavior to be studied is sexual behavior and the animal models to be employed are closely related species of whiptail lizards (Cnemidophorus). One species (C. uniparens) is a triploid parthenogenetic species consisting only of females, while the other species (C. inornatus and C. gularis) are diploid and sexual, consisting of male and female individuals; these latter two species are direct descendants of the evolutionary ancestors of C. uniparens. Behavior patterns that are remarkably similar to the courtship and copulatory behavior of the ancestral sexual species have been documented in C. uniparens as well as other parthenogenetic lizards. This pseudosexual behavior affords a unique opportunity to probe the nature and evolution of sexual behavior. In the parthenogenetic whiptail lizards, gonadal sex and sexual behavior have become uncoupled; in this instance males have been dispensed with, yet the behaviors typical of males have been retained. Because the ancestry of parthenogenetic lizards is known, and because the sexual ancestral species still exist today, comparative studies will provide a probe into the evolutionary constraints on mechanisms controlling species-typical and gender-typical behaviors. Presented in this application are experiments that will determine whether similarities in the form and function of a species-typical behavior reflect a similarity in the underlying physiological mechanism. The first experiments concern the hormonal control of courtship and copulatory behavior in male C. inornatus and of pseudosexual behavior in C. uniparens, and will utilize gonadectomy and replacement therapy techniques. They will be followed by studies on the neural bases of these behaviors employing intracranial implantation, 2-deoxyglucose utilization, and hormone receptor analysis. Other studies will concentrate on the functional significance of pseudosexual behavior and on the sensory modalities that mediate these effects.